Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event

[Sean Christopherson <seanjc@google.com>]

On Wed, Oct 04, 2023, Sean Christopherson wrote:
> Thinking about this more, what if we do a blend of KVM's FPU swapping and debug
> register swapping?
> 
>   A. Load guest PMU state in vcpu_enter_guest() after IRQs are disabled
>   B. Put guest PMU state (and load host state) in vcpu_enter_guest() before IRQs
>      are enabled, *if and only if* the current CPU has one or perf events that
>      wants to use the hardware PMU
>   C. Put guest PMU state at vcpu_put()
>   D. Add a perf callback that is invoked from IRQ context when perf wants to
>      configure a new PMU-based events, *before* actually programming the MSRs,
>      and have KVM's callback put the guest PMU state
> 
> If there are host perf events that want to use the PMU, then KVM will swap fairly
> aggressively and the "downtime" of the host perf events will be limited to the
> small window around VM-Enter/VM-Exit.
> 
> If there are no such host events, KVM will swap on the first entry to the guest,
> and keep the guest PMU loaded until the vCPU is put.
> 
> The perf callback in (D) would allow perf to program system-wide events on all
> CPUs without clobbering guest PMU state.
> 
> I think that would make everyone happy.  As long as our hosts don't create perf
> events, then we get the "swap as little as possible" behavior without significantly
> impacting the host's ability to utilize perf.  If our host screws up and creates
> perf events on CPUs that are running vCPUs, then the degraded vCPU performance is
> on us.
> 
> Rough sketch below, minus the perf callback or any of actual swapping logic.

Another reason to go for an approach that doesn't completely kill off host PMU
usage: just because we don't plan on enable perf events in *production*, there
will undoubtedly be times where we want to enable perf events to debug issues
(outside of prod) in the host kernel/KVM that affect VMs with a passthrough PMU.

So I'll add a self-NAK to the idea of completely disabling the host PMU, I think
that would burn us quite badly at some point.